This invention relates to electrical resistance devices and, more particularly, to devices including a thin layer or film of conductive material on an insulating substrate.
A number of different types of electrical devices are made by depositing a thin film of conductive material, for example, nickel or silver, on an insulating substrate, e.g., paper or organic plastic. The resistivity (ohms per square) of such a layer depends, of course, on the volume resistivity (om-centimeters) of the conductive material and the thickness of the layer. Using vacuum deposition procedures, it is possible to deposit a metal layer as thin as, perhaps, 35 to 40 Angstrom. A nickel layer of such a thickness has a resistivity of about 20 ohms per square.
On a commercial basis it is extremely difficult, if not impossible, to deposit uniform metal films at thicknesses significant less than about 35 Angstroms, and it accordingly also has not been feasible to produce uniform metal layers having a resistivity much greater than that of a uniform 35 Angstrom layer.
It also has been difficult to produce electrical devices in which the resistivity of the metal layer forming one arm of the device is different from that of the metal layer forming another area.